Since GaN has a band gap in the ultraviolet region, the application of GaN as a material for optical devices such as laser devices that generate short-wavelength light such as blue light or ultraviolet light has been promoted. In such optical devices that use GaN, Au (gold), which has low resistance, has been employed as a material for a pad electrode to which an external wiring line (e.g., wire) is connected.
Because GaN has a wide band gap, high carrier mobility, and high critical electric field, attempts to apply GaN to not only optical devices but also power semiconductor devices such as Schottky barrier diodes (SBDs) and transistors have been increasingly made in recent years. In such power semiconductor devices that use GaN, a wiring line (wire) having a large diameter of 200 μm or more and composed of Al (aluminum) is normally used to provide a connection between a power semiconductor device and an external device because a large current needs to be caused to flow.
For example, Japanese Unexamined Patent Application Publication No. 2006-196764 (PTL 1), F. Ren et al., “Wide Energy Bandgap Electronic Devices”, World Scientific, 2003, p. 152-155 (NPL 1), and H. Otake et al., “Vertical GaN-Based Trench Gate Metal Oxide Semiconductor Field-Effect Transistors on GaN Bulk Substrates”, Appl. Phys. Express., 1 (2008) 011105 (NPL 2) disclose structures of conventional power semiconductor devices that use GaN. PTL 1 discloses that a diffusion barrier layer composed of TixW1-xN (0<x<1) is disposed between a low-resistance metal layer and a Ni layer that is in Schottky contact with a compound semiconductor layer composed of GaN. NPL 1 discloses a GaN Schottky barrier diode formed on a GaN substrate. In this Schottky barrier diode, a Schottky electrode is composed of Pt/Ti/Au. NPL 2 discloses a GaN-based vertical MOSFET (metal-oxide-semiconductor field-effect transistor) formed on a GaN substrate. In this MOSFET, a gate electrode is composed of Ni (nickel)/Au.